1. Field of the Invention
The present invention relates to color processing considering chromatic adaptation.
2. Description of the Related Art
Color matching for matching color reproduction between an image display device such as a monitor, projector, or the like and an image output device such as an inkjet or electrophotographic printer is available. There is proposed color matching which converts the absolute value (e.g., the tristimulus value XYZ) of the color of a color patch measured by a measurement device into a color value on a relative color space (color appearance space) considering the color appearance to human, and uses the converted color value.
As a color appearance space, a CIELab space, a JCh space used in CIECAM97s and CIECAM02, or the like is available. In order to convert a measurement value (XYZ value) into that on a color appearance space, a white point (to be referred to as a reference white point, hereinafter) is used as a reference for relative conversion. In general, a color (to be referred to as device white, hereinafter) with a highest brightness which the image display device can display is employed as the reference white point. However, when the color selected as the device white is not an achromatic color but a slightly chromatic color, a phenomenon (incomplete adaptation) occurs in which the human eyes do not completely adapt to the device white. In addition, when viewing a monitor or the like, it is necessary to consider a phenomenon (partial adaptation) in which not only the device white (the white point of the monitor) but also the white point of the light (to be referred to as ambient light, hereinafter) around the viewing environment influences the color appearance.
For example, Japanese Patent Laid-Open No. 9-093451 discloses a technique of calculating a white point in partial adaptation from a plurality of white points to use it as a reference white point.
For example, when the color appearance on a monitor is to be calculated, a white point in the partial adaptation is calculated on a line which connects the white point of the monitor and that of the ambient light. However, since an xy plane, uv plane, or u′v′ plane of a color space used to calculate the white point in the partial adaptation shows poor linearity with respect to the distributions of various actual light sources and the human visual characteristics, an optimal white point in the partial adaptation cannot be calculated.
FIG. 1 is a graph showing the method of obtaining a white point in the partial adaptation. Assume that a monitor with a white point 101 greenish compared to a blackbody locus 104 is viewed under the ambient light with a white point 102 on the blackbody locus 104. In this case, a white point 103 calculated in the partial adaptation is not a white point on the blackbody locus 104 but a somewhat reddish white point.
In other words, as the technique in Japanese Patent Laid-Open No. 9-093451, a technique of calculating a white point in the partial adaptation on a line which connects two different white points so as to set it as a reference white point does not always calculate an optimal white point in partial adaptation.